Given where I drive is often out of range of any data connections my estimate could be wildly misleading. However, I use an AT&T data subscription on my iPad and have traveled across Texas, Oklahoma, New Mexico, and Colorado for days on end and not exhausted the base data download allocation.
The application you are using makes a difference too. Some of them can and will download significant portions, perhaps even all, of the map data while you have a WiFi connection so it is possible you would not use any of your data subscription at all or only very little. Although I have Google Maps my favorite GPS apps are Maps and GPS Drive
. GPS Drive is my "go to" application and that is the one that permits downloading and storing route maps while you are on WiFi.